A recent discovery showed a fluoride-responsive riboswitch class (Weinberg, Z., et al., Genome Biol. 2010, 11, R31; Baker, J. L., et al., Science 2011) present in many bacterial and archaeal species. Riboswitches are metabolite- or ion-sensing domains found within the noncoding portions of certain messenger RNAs where they control the expression of adjoining protein coding regions (Mandal, M. et al., Nature Rev. Mol. Cell Biol. 2004, 5, 451; Roth, A. et al., Annu. Rev. Biochem. 2009, 78, 305; Smith, A. M. et al., RNA Biol. 2010, 7, 104-110). Members of the fluoride riboswitch class bind fluoride anions and regulate numerous genes whose protein products appear to overcome the inherent toxicity of this anion (Baker, J. L., et al., Science 2011).
Although fungi lack representatives of the known fluoride riboswitch class, many fungal species carry a homolog of the gene most commonly associated with fluoride riboswitches in bacteria. This gene (called crcB) codes for a member of a family of proteins predicted to be membrane-associated transporters (Rapp, M. et al., Nat. Struct. Mol. Biol. 2006, 13, 112; Finn, R. D. et al., Nucleic Acids Res. 2010, 38, D211; Holt, R. J. Ann. N. Y. Acad. Sci. 1974, 235, 469). A genetic knock-out of crcB in the bacterium Escherichia coli results in a strain that is approximately 200-fold more sensitive to fluoride, and these cells accumulate higher cytoplasmic concentrations of fluoride compared to wild-type cells when grown in identical fluoride-supplemented growth media (Baker, J. L., et al., Science 2011). Thus, compositions enhancing the toxicity of fluoride are desirable.
Antimicrobial compounds and compositions are well known in the art. Humans can be infected with a diversity of fungal and bacterial species and the outcomes of these diseases can range from minor discomfort and disfiguration to death. Numerous antifungal therapies have been developed over the last several decades that have been very effective (Dismukes, W. E. Clin. Infect. Disease 2006, 42, 1289; Pitman, S. et al., Expert Opin. Emerg. Drugs 2011, 16, 559), but many challenges still exist when treating fungal infections, including the emergence of drug resistance (Ghannoum, M. A.; Rice, L. B. Clin. Microbiol. Rev. 1999, p. 501). Fungal infections on the surface of the body are among the most common (Kaur, I. P.; et al., Expert Opin. Drug Deliv. 2010, 7, 1303) and usually can be overcome by topical treatment with antifungal agents, although poor efficacy can sometimes limit the utility of existing compounds. Fluoride has long been known to inhibit bacterial and fungal cell growth by blocking the functions of key metabolic enzymes. However, a high concentration of fluoride typically is required to provide effective antimicrobial properties.
Other antimicrobial compounds disrupt the integrity of cell membranes which leakage of a variety of small molecules such as potassium and other ion and solute components out of the cell. This disruption in membrane integrity ultimately leads to cell death. One class of such compounds is polyene macrolide antibiotics that selectively inhibit organisms whose membranes contain certain sterols. Their mechanism of action is, at least in part, dependent upon their binding to a sterol moiety, primarily ergosterol, present in the membrane of sensitive fungi. Once this interaction occurs, the polyenes form pores or channels in the fungal cell membrane which results in an increase of permeability of the membrane (WO/2007/096137). Polyene macrolide antifungal agents, such as amphotericin B and nystatin, are well known in the art for the treatment of fungal infections. However, when administered parenterally, such polyene macrolide antifungal agents have a number of serious side effects including nephrotoxicity. These side effects limit the amount of the polyene macrolide antifungal agent that can be administered safely (even topically) to a patient and thus, such side effects limit the effectiveness of these antifungal agents.
Thus, a need exists for new methods and compositions that enhance the efficacy of such antimicrobial compositions thereby permitting a reduced amount of antimicrobial agents to be administered.